Device for amplifying the current of an abnormal electrical discharge and system for using an abnormal electrical discharge comprising one such device

ABSTRACT

A device ( 1 ) for amplifying the current of an abnormal electrical discharge, characterized in that it comprises an electrode which is positively polarized ( 2 ) and associated with a magnetic circuit ( 3 ) producing a magnetic field ( 4 ) which is uniformly divergent, whereby the intensity on the surface of the electrode is more than approximately 6.10 2  Tesla, the electrode being positioned in the region where the magnetic field is at its most intense.

BACKGROUND OF THE INVENTION

The present invention relates to a device for amplifying the current ofan abnormal electrical discharge and a system for using an abnormalelectrical discharge comprising one such device.

The invention relates to all devices for using the current of anabnormal electrical discharge, also known as a luminescent discharge,further known as a cold plasma, in particular for making deposits on,treating and modifying the surface of a metal or non-metal substrate.

Luminescent discharges are increasingly used as a material and/or energytransfer medium in many surface modification processes; processes forcoating metal or non-metal parts may be cited, and among these magnetroncathode sputtering, plasma-enhanced chemical deposition, assistedthermochemical treatment processes such as post-discharge (deferredplasma) or ionic nitriding, ionic etching processes, plasma cleaning,etc. These techniques are used in most industrial fields, especiallymicroelectronics, but also optics and mechanical engineering.

Luminescent discharges (cold plasmas) comprise a plurality of types ofelectrical discharge, each type having specific characteristics. Theperson skilled in the art refers to the electrical discharges to whichthe invention relates as “abnormal discharges”; they are characterizedby the fact that in their domain of existence the current varies as amonotonous function of the excitation voltage.

Devices for using the current of an abnormal electrical discharge totreat a substrate (the term “treat” is to be understood as encompassingdeposition on and modification of the surface of said substrate)generally comprise an enclosure in which a vacuum can be established,since these discharges occur only at reduced pressure, a systemcomprising an electrode (cathode) and an electrical power supply togenerate the discharge, and a substrate carrier designed to support thesubstrates to be treated. The current flowing through the substratedefines the number of ions that bombard it per unit time (ioniccurrent), and the voltage defines the energy of those ions. These twoparameters are important parameters for most processes.

One constraint imposed on such devices is the interdependence of thevoltage and the ionic current.

Attempts to increase the ionic current are the outcome of seeking toimprove the efficiency of the treatment. At low currents, this can beachieved only by increasing the voltage.

However, increasing the voltage can quickly become a problem, as ionswith too much energy can create unwanted modifications of the surface ofthe substrate to be treated, coated or modified.

For example, a number of systems have been described for increasing theflux of ions impinging on the substrates to be treated when themagnetron cathode sputtering technique is used. The most widely used isthe unbalanced magnetron cathodes technique. The magnetic configurationof the system forces the energetic electrons fleeing the magnetic trapof the cathode to converge toward the substrate, where they increase theionization of the gas. The flux of ions obtained depends on thecharacteristics of the electrical discharge. Such systems have beendeveloped in particular by B. Window (see the papers by B. Window and N.Savvides, J. Vac. Sci. Technol. A 4 (3) May/June 1986, pages 453–456“Unbalanced dc magnetrons as sources of high fluxes” and J. Vac. Sci.Technol. A 4 (2) March/April 1986, pages 196–202 “Charged particlefluxes from planar magnetron sputtering sources”).

Devices for amplifying the current of an unbalanced magnetron cathodeselectrical discharge cannot increase the ionic current on the substratewithout in parallel increasing the ion acceleration voltage.

Systems have also been developed using auxiliary systems, for examplefrom magnetron cathodes to microwave excited plasma systems.

Systems have also been developed using a filament and a hollow cathode.

Such systems are not in widespread use today, mainly because they aredifficult to use and unreliable.

Furthermore, the prior art systems cannot, without great difficulty ifat all, be produced with different geometries and different sizes, inorder to cater for a large number of industrial configurations.

SUMMARY OF THE INVENTION

One object of the invention is to provide a system for using the currentof an abnormal electrical discharge for treating a substrate whichincreases the ionic current on the substrate without in parallelincreasing the ion acceleration voltage.

Another object of the invention is to provide a system that is reliableand easy to use for using the current of an abnormal electricaldischarge to treat a substrate.

A further object of the invention is to provide a system for using thecurrent of an abnormal electrical discharge that can easily beimplemented in a plurality of generic geometries, for example circularor rectangular, and with various dimensions.

Other objects and advantages of the invention will become apparent onreading the following description.

To meet the above objects, the present invention provides a system forusing an abnormal electrical discharge which includes a plasma sourceand is characterized in that it includes a new auxiliary device. Theauxiliary device is based on using combined electric and magnetic fieldswhich, coupled with a plasma source, considerably increases the ioniccurrent on the substrates independently of the ion acceleration voltage.A device of the above kind is referred to hereinafter as a device foramplifying the current of an abnormal electrical discharge, an ionicamplifier or a plasma amplifier.

A device in accordance with the invention for amplifying the current ofan abnormal electrical discharge is characterized in that it comprisesan electrode which is positively polarized and associated with amagnetic circuit producing a magnetic field which is uniformlydivergent, whereby the intensity on the surface of said electrode ismore than approximately 6.10⁻² Tesla, said electrode being positioned inthe region where the magnetic field is most intense.

The magnetic field is measured at the surface and at the center of theionic amplifier.

The magnetic field preferably has an intensity from approximately 8.10⁻²Tesla to approximately 25.10⁻² Tesla and even more preferably fromapproximately 10.10⁻² Tesla to approximately 15.10⁻² Tesla.

Too low a magnetic field produces an unstable plasma whereas too high amagnetic field can be dangerous without being more effective.

The magnetic circuit can be made up of permanent magnets and/orelectromagnets, for example.

The dimensions of the device in accordance with the present inventionfor amplifying the current of an electrical discharge are related to theconfiguration of the utilization system in which it is installed. Thepositively polarized electrode can be a disk whose diameter is fromapproximately 50 mm to approximately 200 mm, for example.

More generally, the positive electrode has a rectangular shape whosewidth is from approximately 50 mm to approximately 200 mm and whoseheight is from approximately 200 mm to approximately 1000 mm, forexample.

Although this is not limiting on the invention, the plasma amplifierpreferably includes a metal counter-electrode for stabilizing theplasma.

The counter-electrode is a metal component that can be either connectedto ground or biased to a voltage from 0 to approximately −200 V relativeto ground.

The counter-electrode is positioned on the path of the electricaldischarge.

The person skilled in the art will readily be able to determine theposition of the counter-electrode.

The counter-electrode must be substantially parallel to the surface ofthe amplifier, preferably at a distance from approximately 5 mm toapproximately 70 mm from the surface of the positively polarizedelectrode of said amplifier.

The counter-electrode can comprise a plate incorporating an orifice, forexample a rectangular hole. The person skilled in the art willunderstand that the dimensions of the orifice must be such that thedivergent magnetic field lines are not intercepted by the plate.

The counter-electrode can instead comprise a grid.

Finally, the counter-electrode can instead comprise a rod, for example arod with a diameter from 5 mm to 20 mm. In this case the rod isadvantageously parallel to the central magnetic pole.

Depending on the embodiment of the invention, it may be necessary tocombine more than one type of counter-electrode in the same amplifier.

The device in accordance with the invention for amplifying the currentof an electrical discharge can include a plurality of counter-electrodesof different types or the same type.

The counter-electrodes can be of the plate type, of the rod type, of thedisk type, of the rectangular type or of the grid type, for example.

The present invention also provides a system for using an abnormalelectrical discharge, including a plasma source in an enclosurecontaining a gas at reduced pressure, characterized in that it includesin said enclosure a device in accordance with the invention as describedabove for amplifying the current of an electrical discharge.

The plasma sources are also referred to as “primary sources”.

The system according to the invention can include a plurality of plasmasources. In this case, the system includes a plurality of devices foramplifying the current of an electrical discharge. Although this is notlimiting on the invention, each of the devices for amplifying thecurrent of an electrical discharge is preferably associated with onlyone plasma source.

The system according to the invention for using an electrical dischargecan be used to make a deposit on, treat or modify the surface of asubstrate, for example. These uses are referred to by the generic term“treatment” in the description.

In a preferred embodiment of the invention the plasma source or at leastone of the plasma sources is a magnetron cathode sputtering cathode.

In this embodiment, if the positively polarized electrode of the devicefor amplifying the current of an electrical discharge is rectangular,the length of the positively polarized electrode must be substantiallythe same as that of the magnetron cathode, in order to obtain ahomogeneous plasma in the treatment area.

A system according to the invention for using an abnormal electricaldischarge generally includes a rotatable substrate carrier designed tosupport the parts to be treated. The substrate carrier can itself benegatively polarized to accelerate the positive ions emanating from thesource and thus bombard the substrate.

The disposition of the device for amplifying the current of an abnormalelectrical discharge (plasma amplifier) must be such that the amplifiedplasma diffuses toward the substrate carrier. Also, it is preferable tominimize the distance between the primary source and the plasmaamplifier. Accordingly, in small deposition machines (diameter<400 mm),the dimensions are sufficiently small to position the plasma amplifieranywhere provided that the substrate carrier does not screen in apenalizing manner the electrons emanating from the primary source. Inlarger machines (diameter>400 mm), the plasma amplifier isadvantageously disposed at a distance of less than 400 mm from theprimary source and this distance is preferably reduced to the minimumpermitted by the mechanical construction. This configuration allowssatisfactory operation of the plasma amplifier because the concentrationof electrons emanating from the source is sufficient to ignite theamplified plasma. Also, this configuration minimizes interaction withthe substrate carrier. The electrons can then pass directly from thesource to the device for amplifying the current of an electricaldischarge.

As a general rule, the direction of the magnetic polarity of the devicefor amplifying the current of an electrical discharge has no significanteffect on its effectiveness. However, when the source uses devices basedon magnetic fields to generate the plasma, the magnetic polarity of theamplifier must be adjusted relative to the primary source, especially ifthe primary source is a magnetron cathode sputtering cathode. Thecentral pole of the magnetron cathode and the device for amplifying thecurrent of an electrical discharge must preferably have the same sign,so that no looping of field lines is possible between the cathode andthe device for amplifying the current of an electrical discharge. It isincreasingly important to comply with this feature if the distancebetween the device for amplifying the current of an electrical dischargeand the magnetron cathode decreases (d<200 mm). If the distance is large(from 200 mm to 400 mm), the direction of the magnetic polarity is nolonger important, because the interaction of the magnetic fields is thennegligible.

The present invention further provides a method of treating parts(substrates) with a plasma, characterized in that it uses the systemdescribed above for using an abnormal electrical discharge.

In a method according to the invention, the electrical power supplywhich supplies power to the device for amplifying the current of anelectrical discharge must have a minimum voltage for the amplificationmechanism to be initiated. The positive voltage must be sufficient (inpractice greater than approximately 20 V) for the electrons to be ableto ionize the gas. When amplification commences, the positive voltage ofthe amplifier changes very little when the imposed current changes overa large range. Depending on what is required, the power supply canprovide a direct current or a current chopped at a low, medium or highfrequency. If the source is a magnetron cathode sputtering cathode, themaximum current imposed on the plasma amplifier must not exceed 1.5times that of the source, regardless of the mode of polarization of thelatter. Beyond 1.5 times the cathode current, the amplified plasmabecomes very unstable and generates electrical arcs on the variouscomponents of the deposition system, and the voltage of the amplifierbegins to increase greatly (greater than approximately 60 V) andfluctuates intensely. Moreover, if the voltage of the plasma amplifierbecomes too high (>60 V), the walls of the deposition machine begin tobe pulverized, which degrades the quality of the process.

A second feature of the method according to the invention isconsequently that the supply voltage of the device for amplifying thecurrent of an electrical discharge must be from approximately 20 voltsto approximately 60 volts.

The plasma amplifier according to the invention has many advantages. Inparticular, the plasma amplifier increases the ionic current on thesubstrate carrier by a factor from 1 to 10 compared to the currentnormally obtained with conventional magnetron cathode sputtering. Thefactor depends on the current imposed on the amplifier (from 0 to 1.5times the cathode current).

The invention also has other advantages that are particularly beneficialfor the user: the ionic current from the plasma amplifier can beadjusted independently of that from the primary source. Also, the biasvoltage and the current of the substrate carrier can be adjustedindependently, so that it is possible to control independently thenumber and the energy of the ions bombarding the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail next, but without limiting theinvention, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of a device according to the invention foramplifying the current of an abnormal electrical discharge,

FIG. 2 is a diagrammatic view of an embodiment of a device according tothe invention for amplifying the current of an abnormal electricaldischarge including a counter-electrode in the form of a plate, and

FIG. 3 is a diagrammatic view of an embodiment of a device according tothe invention for amplifying the current of an abnormal electricaldischarge including a counter-electrode in the form of a rod.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the device 1 in accordance with the invention foramplifying the current of an abnormal electrical discharge includes arectangular electrode 2 which is positively polarized, associated with amagnetic circuit 3 producing a magnetic field 4 which is uniformlydivergent, whereby the intensity on the surface of said electrode ismore than approximately 6.10⁻² Tesla. The electrode 2 is positioned inthe region where the magnetic field 4 is most intense.

Referring to FIG. 2, the device 1 according to the invention foramplifying the current of an electrical discharge further includes ametal counter-electrode 5 in the shape of a plate positioned on the pathof the electrical discharge and substantially parallel to the surface ofthe amplifier device 1.

Referring to FIG. 3, the device 1 according to the invention foramplifying the current of an electrical discharge further includes ametal counter-electrode 5′ in the form of a rod positioned on the pathof the electrical discharge and substantially parallel to the surface ofthe amplifier device 1.

The following example serves to illustrate the invention withoutlimiting it.

EXAMPLE

In a 350 mm diameter vacuum deposition machine, a 150 mm diametermagnetron cathode is installed and supplied with a power of 1800 W at avoltage of 550 V (3.3 A). The plasma amplifier receives a supply currentof 4 A. The rotating substrate carrier is biased to a voltage of −100 V.The bias current Ib is measured.

The magnetic field at the surface of the amplifier is 11. 10⁻² Tesla.

The electrode is a 150 mm diameter disk.

The ion acceleration voltage V is measured with and without the plasmaamplifier according to the invention (configuration 0 and configuration1, respectively).

The efficacy of the system is characterized by the factor k, the ratioof the current Ib_(A) collected on the parts when the plasma amplifieris operating and the current Ib received when there is no plasmaamplifier.

The results are indicated in Table I below.

TABLE I Plasma V in volts amplifier on substrate Configuration present k= Ib_(A)/Ib carrier 0 (comparative) No 1 −100 1 Yes 6.2 −100

Note that the plasma amplifier according to the invention increases theionic current on the substrate without in parallel increasing the ionacceleration voltage V.

The person skilled in the art will understand that although theinvention has been described and shown by means of particularembodiments, numerous variants can be envisaged without departing fromthe scope of the invention as defined in the accompanying claims.

1. A device (1) for amplifying the current of an abnormal electricaldischarge, wherein the device is arranged to amplify the current of theabnormal electrical discharge at a substrate being treated and includesan electrode disk (2) which is positively polarized and associated witha magnetic circuit (3) producing a magnetic field (4) which is uniformlydivergent, whereby the intensity on the surface of said electrode ismore than approximately 6.10⁻² Tesla, said electrode being positioned inthe region where the magnetic field is most intense.
 2. A device (1)according to claim 1 for amplifying the current of an abnormalelectrical discharge, characterized in that the magnetic field (4) hasan intensity from approximately 8.10⁻² Tesla to approximately 25.10⁻²Tesla.
 3. A device (1) according to claim 2 for amplifying the currentof an abnormal electrical discharge, characterized in that the magneticfield (4) has an intensity from approximately 10.10⁻² Tesla toapproximately 15.10⁻² Tesla.
 4. A device (1) according to claim 1 foramplifying the current of an abnormal electrical discharge,characterized in that the disk has a diameter from approximately 50 mmto approximately 200 mm.
 5. A device (1) for amplifying the current ofan abnormal electrical discharge, wherein the device includes anelectrode (2) which is positively polarized and associated with amagnetic circuit (3) producing a magnetic field (4) which is uniformlydivergent, whereby the intensity on the surface of said electrode ismore than approximately 6.10⁻² Tesla, said electrode being positioned inthe region where the magnetic field is most intense, and a metalcounter-electrode (5, 5′) positioned on the path of the electricaldischarge and substantially parallel to the surface of said electrode.6. A device (1) according to claim 5 for amplifying the current of anabnormal electrical discharge, characterized in that thecounter-electrode (5, 5′) is positioned at a distance from approximately5 mm to approximately 70 mm from the surface of the positively polarizedelectrode of said amplifier device.
 7. A device (1) according to claim 5for amplifying the current of an abnormal electrical discharge,characterized in that the counter-electrode is a plate.
 8. A device (1)according to claim 7 for amplifying the current of an abnormalelectrical discharge, characterized in that the plate has an orificethrough it.
 9. A device (1) according to claim 8 for amplifying thecurrent of an abnormal electrical discharge, characterized in that theorifice is rectangular.
 10. A device (1) according to claim 5 foramplifying the current of an abnormal electrical discharge,characterized in that the counter-electrode is a rod.
 11. A device (1)according to claim 10 for amplifying the current of an abnormalelectrical discharge, characterized in that the rod has a diameter fromapproximately 5 mm to approximately 20 mm.
 12. A device (1) according toclaim 5 for amplifying the current of an abnormal electrical discharge,characterized in that the counter-electrode is a disk.
 13. A device (1)according to claim 5 for amplifying the current of an abnormalelectrical discharge, characterized in that the counter-electrode isrectangular.
 14. A device (1) according to claim 5 for amplifying thecurrent of an abnormal electrical discharge, characterized in that thecounter-electrode is a grid.
 15. A device (1) according to claim 5 foramplifying the current of an abnormal electrical discharge,characterized in that it includes a plurality of counter-electrodes. 16.A device (1) according to claim 15 for amplifying the current of anabnormal electrical discharge, characterized in that thecounter-electrodes are of different types.
 17. A device (1) according toclaim 16 for amplifying the current of an abnormal electrical discharge,characterized in that the counter-electrode types are chosen from theplate type, the rod type, the disk type and the grid type.
 18. A systemfor using an abnormal electrical discharge, including a plasma source inan enclosure containing a gas at a reduced pressure, wherein the systemincludes in said enclosure a device according to claim 1 that isseparate from said enclosure and said plasma source.
 19. A systemaccording to claim 18 for using an abnormal electrical discharge,characterized in that it includes a device for amplifying the current ofan abnormal electrical discharge for the plasma source or for eachplasma source, the or each device for amplifying the current of anelectrical discharge being associated with only one plasma source.
 20. Asystem according to claim 18 for using an abnormal electrical discharge,characterized in that at least one of the plasma sources is a magnetroncathode sputtering cathode.
 21. A system according to claim 18 for usingan abnormal electrical discharge, characterized in that said use ischosen from deposition on and treatment and modification of the surfaceof a substrate.
 22. A system according to claim 21 for using an abnormalelectrical discharge, characterized in that it is associated with arotatable substrate carrier.
 23. A system according to claim 22 forusing an abnormal electrical discharge, characterized in that thesubstrate carrier is negatively polarized.
 24. A method of treating thesubstrate with a plasma, characterized in that a system according toclaim 18 for using an abnormal electrical discharge is used and in thatthe positive supply voltage of the device for amplifying the current ofan abnormal electrical discharge is from approximately 20 volts toapproximately 60 volts.